The usefulness of an intrusion detection system critically depends on its capability to distinguish an alarm condition initiated by an actual unauthorized intruder from either a false alarm or a failure of alarm produced by noise, some atmospheric disturbance, an animal, alterations in the placement and state of operability of protected area equipment, and change in actual versus the design range, among other things. Ultrasonic intrusion detection systems are not only subject to false alarm indications in response to drafts and air movements as well as in response to ultrasonic noises such as caused by bells and hissing but also are subject to failure of alarm situations in response to changes from nominal range occasioned by variations in the ultrasonic propagation medium. Microwave intrusion detection systems produce false alarms in response to water movement in plastic pipes, in response to energy received from beyond the protected area due to wall and window penetration, and also in response to unwanted reflections, among other things.
The sources that adversely affect the performance of ultrasonic detection systems are in general different from those that give rise to false alarms and failures of alarm for microwave detection systems, and conversely. Thus, while drafts, air movements, and ultrasonic noises adversely affect ultrasonic system performance, none of them poses a significant detection problem for microwave systems. Similarly, while water movement in plastic pipes, wall or window penetration, and reflections give rise to false alarms for microwave intrusion detection systems such events are not obstacles to accurate detection for ultrasonic systems.
Galvin, U.S. Pat. No. 4,195,286, issued Mar. 25, 1980 and assigned to the same assignee as the instant invention, discloses a combination detection system providing both a high and a low probability system output signal. The high probability output signal system is produced by a logical AND circuit coupled to the output of at least two independent sensors that is only operative in response to the simultaneous presence of the several sensor outputs. The occurrence of any one of the error producing sources for less than all of the independent sensors thus fails to produce a system alarm signal indication. The exclusive occurrence, then, of any one of the sources that give rise to ultrasonic false detections such as bells or hissing, or the exclusive occurrence of any one of the microwave system error sources such as water movement in plastic pipes, fails to enable the AND gate and inhibits the production of an erroneous system alarm signal indication.
Combination detection systems based on an AND detection principle produce a system alarm signal whenever its inputs are simultaneously present irrespectively of whether the inputs represent true intruder motion or arise from error producing sources, so that they often produce an erroneous alarm indication. Such an event could occur, for example, if a microwave system in penetrating the walls of a protected area returns an indication of someone present on the street beyond, while an ultrasonic system simultaneously produces an erroneous indication of intruder presence in response to an air conditioner being turned on in the protected area.
Other disadvantages of combined detection systems using "AND" detection verification include the necessity for controlling the range of the microwave and ultrasonic systems separately, the need for separately walk-testing each of the systems individually, and the fact that the combined system degrades to that one of its independent sub-systems not subject to the error producing condition. Reference may be had to U.S. Pat. Nos. 3,074,054, and 3,074,053, for other combination detection systems based on an "AND" function detection principle.